One of the most abundant and economically viable alternatives to oil-derived fuels such as gasoline, kerosene and diesel is natural gas. Because of this, engine manufacturers for transport and stationary applications are shifting their attention from the traditional oil-derived fuels to the cheaper, cleaner burning, and more environmentally friendly compressed natural gas (CNG) or liquefied natural gas (LNG) fuels. In recent years, significant investments and efforts have been made to expand the natural gas fuel supply chain/infrastructure and develop natural gas specific engine hardware in order to enable the broad deployment of natural gas as s fuel. The major component in natural gas is methane. The exhaust gas of a natural gas fueled engine may contain some residual methane, which is preferably removed before the exhaust gas is released into the atmosphere in order to meet current and future environmental emission regulations. One way of removing the residual methane in the exhaust gas is by catalytically oxidizing the methane to carbon dioxide and water. Catalysts in current day catalytic converters used to treat exhaust gas are not designed to convert methane. Due to the relatively high activation temperature required for the combustion of methane, methane will typically pass through such catalytic converters unconverted.
Catalysts for methane oxidation have been reported previously. In WO2009/057961 a catalyst containing palladium and platinum supported on alumina for treating exhaust gas from a dual fuel, i.e. diesel and LNG, fueled vehicle is disclosed. The catalysts are said to have a preferred palladium:platinum ratio of 1.0:0.1-0.3 and are deposited on an alumina support. However, the performance advantage of these palladium-platinum/alumina materials was demonstrated without the presence of H2O in the feed. It is well known that exhaust gases from natural gas fueled engines in transport and stationary applications contain very high levels of H2O usually in the range of 9-17 vol %. These significant H2O levels in exhaust gas are known to have a very significant adverse effect on the activity of palladium-platinum/alumina catalysts and the stability of these catalysts in the methane oxidation reaction. Therefore, it is expected that these prior art alumina-based catalysts will suffer from excessive activity loss and activity decline rates in a commercial application for conversion of methane in exhaust gases containing significant levels of water.
U.S. Pat. No. 5,741,467 discloses mixed palladium/alumina and palladium/ceria/lanthana alumina wash coat formulations used as methane oxidation catalysts for fuel-lean or fuel-rich methane oxidation, respectively. In addition, U.S. Pat. No. 5,741,467 discloses that rhodium may be used to substitute completely or in part for the palladium. However, these catalytic formulations exhibit very low methane oxidation activity after aging as illustrated by their high temperature requirements (higher than 500° C.) for 50 vol % methane conversion, generally referred to in the art as the T50 (CH4). The low methane oxidation activities and rapid activity declines exhibited by these catalysts suggest that these catalytic formulations would most likely not find acceptance or utility in commercial natural gas-fueled engine exhaust gas treatment applications.
U.S. Pat. No. 660,248 discloses catalysts for removing hydrocarbons from exhaust gas containing methane and excess of oxygen. The catalysts comprise palladium or palladium/platinum supported on at least one support selected from zirconia, sulfated zirconia and tungsten-zirconia. The disclosed zirconia-based catalysts show improved performance compared to the previously discussed alumina-based catalyst with respect to methane oxidation activity, however the activity of these catalysts is still too low to be attractive for commercial application.
Therefore, there is a need for methane oxidation catalysts that exhibit higher methane oxidation activity for the efficient removal of non-combusted methane from exhaust gas from natural gas-fueled engines.